1. Field of the Invention
The present invention relates to a liquid crystal display comprising (1) a liquid crystal display panel (hereinafter, referred to also as “LCD panel”) which comprises a backlight, a liquid crystal (liquid crystal panel) and its driver and (2) an image data processing device for generating corrected image data from raster data inputted from the outside, which is used to determine a voltage to be applied to the liquid crystal of the LCD panel, and more particularly to a technique for processing image data for the LCD panel to optimize a response speed of the liquid crystal (which corresponds to the amount of change in transmittance of the liquid crystal per unit time) in accordance with a change in luminance of a moving image to be inputted.
2. Description of the Background Art
Since transmittance of a liquid crystal varies depending on a cumulative response effect, an LCD panel involves a problem that it can not appropriately respond to an inputted moving image with luminance variation which is relatively faster in speed than a response of the liquid crystal. In order to solve this problem, proposed is a method for improving the response speed of the liquid crystal in which a driving voltage of the liquid crystal at the time of change in luminance of the inputted moving image is intentionally made larger than a normal driving voltage.
An exemplary liquid crystal display device which is made capable of controlling its response speed by a liquid crystal driving operation through the above method so that the response speed of the liquid crystal should increase in accordance with change in luminance of the inputted moving image is disclosed in detail in Japanese Patent No. 2616652 (referred to as a “first prior art”). Specifically, the liquid crystal display device disclosed in the first prior art comprises an A/D converter circuit for sequentially A/D converting raster image data which give pixels in each of motion screens, an image memory (frame memory) for holding the image data for one frame of the inputted motion screen, a comparator circuit for comparing present image data of a pixel with one-frame preceding image data of the pixel to output a luminance change signal, a driving circuit for liquid crystal panel and a liquid crystal panel.
Next, an operation of the above liquid crystal display device will be discussed. The A/D converter circuit samples the raster image data of analog format with a sampling clock having a predetermined frequency, converts the sampled raster image data into image data of digital format and outputs the converted image data to the image memory and the comparator circuit. The image memory reads one-frame preceding image data which is stored in advance at an address corresponding to the pixel in response to the input of the image data of each pixel to output the one-frame preceding image data to the comparator circuit and overwrites the inputted present image data at the above address. Thus, the image memory serves as a delay circuit for delaying the present image data of each inputted pixel by a period corresponding to one frame. The comparator circuit compares the present image data outputted from the A/D converter circuit with the one-frame preceding image data outputted from the image memory to output a luminance change signal which gives a luminance change of the image between the present image data and the one-frame preceding image data, together with the present image data, to the driving circuit. The driving circuit applies a driving voltage higher than a normal liquid crystal driving voltage to the liquid crystal panel with respect to the pixel whose luminance value increases, on the basis of the luminance change signal, thereby to drive the display pixel on the panel. On the other hand, the driving circuit applies a driving voltage lower than the normal driving voltage to the liquid crystal panel with respect to the pixel whose luminance value decreases, on the basis of the luminance change signal, thereby to drive the display pixel on the panel.
In the liquid crystal display device disclosed in the first prior art, however, when the number of pixels of the liquid crystal panel becomes larger, since the number of image data for one frame to be written in the image memory accordingly increases, the memory capacity required as the image memory inevitably becomes larger.
Then, from the viewpoint of reduction in capacity of the image memory, a liquid crystal display device disclosed in Japanese Patent No. 3041951 (referred to as a “second prior art”) proposes a skipping operation method where one address of the image memory is allocated to four pixels. Specifically, in the second prior art, alternate ones of the pixel data arranged in matrix are skipped in each of the horizontal and vertical directions and each of the remaining image data is stored in the image memory, and in read operation from the image memory, for the three adjacent skipped pixels, the same image data as the image data of the corresponding stored pixel is read out three times, allocating the skipped pixel image data, to reduce the memory capacity of the image memory. For example, when the image data of a pixel at coordinates (a, A) is stored at address 0 in the image memory, the image data at the address 0 is read and allocated to the three skipped pixels at coordinates (a, B), (b, A) and (b, B).
When the method proposed in Japanese Patent No. 3041951 is used, however, the following problem is caused, instead. This problem will be shown in FIGS. 46A to 46D (non prior arts).
FIG. 46A shows image data in an n-th frame, FIG. 46B shows the image data obtained after the skipping operation for the image in the n-th frame shown in FIG. 46A, FIG. 46C shows the image data obtained after interpolation by the above read operation of the skipped pixel data, and FIG. 46D shows the image data in an (n+1)-th frame posterior to the n-th frame by one frame. As shown in FIGS. 46A and 46D, the image in the n-th frame and that in the (n+1)-th frame are equal to each other.
When the skipping operation is performed, as shown in FIG. 46C, the pixel data at (A, a) is read out as the pixel data at (B, a) and (B, b) and the pixel data at (A, c) is read out as the pixel data at (B, c) and (B, d). Specifically, the pixel data which actually has a luminance value of 150 is read out as the pixel data which has a luminance value of 50. Therefore, though there is no change between the image in the present frame and the one-frame preceding image, the respective display pixels corresponding to the addresses (B, a), (B, b), (B, c) and (B, d) in the n-th frame are driven by a driving voltage higher than a normal driving voltage.
Thus, when the skipping operation is performed, a correct control of the voltage is not made at a portion where the pixel data is skipped and as a result, deterioration in image quality occurs due to an unnecessary voltage which is applied.
As discussed above, in these prior-art patent inventions, even if there is a change (difference) in luminance value between the present frame and the one-preceding frame, it is possible to improve the response speed of the liquid crystal by setting the liquid crystal driving voltage larger than a normal driving voltage.
The former prior-art patent invention (the first prior art), however, has a problem of causing an increase in capacity of the image memory which has a delay function, and the latter prior-art patent invention (the second prior art) has a problem that deterioration in image quality is caused by reduction in memory capacity, and therefore both prior arts have their respective merits and demerits.